This invention relates to projection display systems which use reflective spatial light modulators, and more specifically, to such systems which use two polarization states of light to produce color images.
The system shown in FIG. 1 illustrates the essential components of the optical portion of a conventional projection display system having three reflective spatial light modulators in the form of liquid crystal display (LCD) panels, also known as liquid crystal light valves (LCLV). The prior art system, depicted generally at 10 includes a light source 12, an illumination mechanism for collecting the light and concentrating it onto the light valves, shown generally at 14, a polarizing mechanism for polarizing the light, if the light valves modulate via polarization effects, shown generally at 16, a splitting mechanism for splitting the illumination into three color bands to separately illuminate the three light valves, shown generally at 18, a recombining mechanism for recombining the three light distributions after reflecting from the light valves, shown generally at 20, and a projection mechanism for projecting the combined images onto a viewing screen, shown generally at 22.
Lamp 24 and lamp reflector 26 produce and concentrate the light for this system. A series of dichroic filters 28, 30 is used to split the light from the lamp 24 into separate red, green, and blue components. The light in each of the three components, or channels, is then polarized with a polarizing beam splitter (PBS) 32, 34, 36, and illuminates three separate LCDs, 38, 40, 42. The LCDs selectively modify the polarization of the light reflected from them allowing some portion of the light to pass back through the PBS. A second series of dichroic filters, 44, 46, is used to recombine the modulated light distributions and pass them on to a projection lens 48 imaging all three LCDs onto the viewing screen.
The configuration shown in FIG. 1 is functional and has been used to implement projection display system products. However, one drawback of such systems is that they are inefficient, utilizing only one half of the light from the light source. This is a particular problem since the displays tend to be dim and it is desired to produce bright images using such displays. Polarization converters have been used to improve optical throughput by converting the randomly polarized light from a light source to a single polarization state. However, such polarization converters are not entirely efficient and can introduce depolarization effects that can effect contrast, and also add additional complexity and cost.
Another drawback of such systems is that color balance is often sacrificed to improve brightness of the projected image. For color displays, one aspect of picture quality is color temperature. This is a subjective evaluation, indicated by the "whiteness" of white. It is assumed by analogy to the photographic film industry that color temperature preferences follow certain geographic patterns. For example, Americans seem to prefer a bluish white color temperature, while Europeans seem to prefer whatever color temperature provides a "truest" skin tone. It is desirable for a color display system to be able to provide whatever color temperature is preferred in a given market. Color balance has been achieved conventionally by providing additional filtering to decrease the intensity of particular color components, thus correcting any imbalance in the light source. However, because image brightness is already a problem in conventional display systems, it is often undesirable to further decrease brightness in order to achieve a more desirable color temperature.
Yet another drawback of the prior art projection display systems is that the large number of components in the architecture shown in FIG. 1 is cumbersome, and necessitates a relatively large physical size of the system. Still another drawback to these systems is the requirement of a large back working distance for the projection lens.
The prior art projection display systems which have sought to provide full color images have not adequately addressed these drawbacks. Accordingly, there is still a need for a color projection display that efficiently transmits light of both polarization states through the projection display system to yield a bright image, that achieves the desired color temperature but that does not suffer from depolarization effects, that utilizes a small number of components in a small physical size of, the system, and that has a relatively short back working distance for the projection lens.